Until now most circuit substrates have been made of ceramic substrates such as Al.sub.2 O.sub.3 substrate or resin substrate. In particular, Al.sub.2 O.sub.3 substrate has superior mechanical strength and electrical insulation properties and in addition can be easily made into green sheets, which makes high-density wiring such as multi-layered wiring possible, so it has found wide use. However, the thermal conductivity of Al.sub.2 O.sub.3 is low, only about 20 W/mK.
In recent years, as electronic devices have become smaller, the packaging density of electronic devices (such as ICs) mounted on a circuit substrate has increased. In addition, power semiconductors are being used. Consequently, a great deal of heat is produced by the electronic devices, making it necessary for the circuit substrate to radiate heat away efficiently. However, since the thermal conductivity of Al.sub.2 O.sub.3 is low, when a great deal of heat is produced, it is impossible to expect much of it to be radiated away from the circuit substrate. Consequently, when mounting electronic devices in a high density packaging configuration or producing modules containing power semiconductors, it is desirable to have a circuit substrate which has high thermal conductivity in addition to mechanical strength and good electrical insulation.
Meanwhile, in recent years, with progress in fine ceramics technology, ceramic materials such as SiC and AlN having superior mechanical strength have been developed. These materials have superior thermal conductivity and much research has been done on applying them as structural materials. Since SiC with 0.5 to 3 wt % BeO has good thermal conductivity, there are some efforts being made to use SiC as a circuit substrate material. However, SiC has a high dielectric constant and low dielectric strength, so there are serious problems in using it for high-frequency circuit devices and for devices to which high voltage will be applied.
Aluminum nitride (AlN) is improved over the alumina ceramic with respect to characteristics required as circuit substrates because it has an outstandingly high thermal conductivity of at least 40 W/mk, for example, 100 W/mk as compared with 20 W/mk for alumina, a mechanical strength of 40 to 50 kg/mm.sup.2 (25 kg/mm.sup.2 for alumina), and a dielectric strength of 140 to 170 kV/cm (100 kV/cm for alumina). It is desired to manufacture high thermal conductivity circuit substrates by taking advantage of such favorable characteristics of AlN ceramic.
Because of good electric insulation as well as thermal conductivity, AlN substrates would find a promising application as circuit substrates. AlN was, however, believed difficult to bond a conductive layer thereto because of its poor wettability by metal as evidenced by its use as crucibles for melting metallic aluminum. There is no circuit substrate having a conductive layer formed directly on an AlN substrate. AlN ceramic has found the only application as a heat sink on which power semiconductors such as thyristors are secured with organic adhesive.
Since AlN ceramic is hardly adhered to metals, it is difficult to use such as copper, silver, silver-platinum etc. on the ceramic for a circuit substrate.
Japanese Patent Publication of Unexamined Application Nos. 52-37914 and 50-132022 disclose techniques of directly bonding a copper plate to ceramic. Although a conductive layer might be formed on an AlN substrate using these techniques, a limitation is imposed on the formation of a fine pattern and great difficulty is encountered in providing multilayer wiring necessary in high density packages.
In addition, AlN ceramic has poor wettability by glass, so that when a glass layer is formed directly on an AlN substrate without an intervening oxide layer which is described hereinafter, bubbles are introduced into the glass layer so that a strong bond cannot be established. It is believed that bubble formation is due to evolution of gases such as ammonia gas from the AlN substrate heated to elevated temperatures in air.
The thick film pastes which may be used in circuit substrates include a variety of pastes commonly used in forming conductive and resistive layers and containing as a conductor a metal, metal oxide and the like, for example, Au, Ag, Pt, Cu, Ni, Ru, RuO.sub.2, Bi.sub.2 Ru.sub.2 O.sub.7 etc. Examples of the conductive pastes which may be used include usual Ag, Au, Cu, Ni, Al pastes. Examples of the resistive pastes which may be used include RuO.sub.2 paste, LaF.sub.6, YB.sub.6, CaB.sub.6, BaB.sub.6, SrB.sub.6 paste, and the like.
It is also possible to use a dielectric paste consisting mainly of lead borosilicate glass (PbO-SiO.sub.2 -B.sub.2 O.sub.3) to form the dielectric layer for multi-layered wiring. It is desirable for such a glass compound to contain PbO to increase the bond strength.
The thick film pastes generally contain a solvent for slurry formation, a binder and a component contributing to bonding in addition to the conductive powder as a main component.
Since a pattern of electrically conductive paths or layers can be formed from such a thick film paste, finely detailed circuit design becomes possible. In addition, when devices such as power semiconductors producing a great deal of heat are to be used, it is possible to, for example, form the principal electrically conductive paths or layers from thick film paste and connect the devices to, for example, a copper plate bonded to the AlN substrate. Such bonding can, for example, be performed using technology in Patent Publication of Unexamined Application Nos. Sho-52-37914 and Sho-50-132022.
The bond structures to ceramic substrate include the chemical bond type where a chemical bond is formed, the glass bond type based on glass adhesion, and the mixed bond type which is a mixture of these types. The thick film pastes for use in circuit substrates are of the chemical bond type or based on the chemical bond type and basically do not contain a glass component although a glass component may be contained as long as it does not adversely affect the chemical bond. When the glass component is contained in a paste, cracks and other defects readily occur during baking due to the difference in coefficient of thermal expansion, bonding force is lessened, and electric resistance is increased.